Method and apparatus for a high voltage power supply circuit

ABSTRACT

A high voltage power supply method and apparatus is disclosed. An apparatus according to aspects of the present invention includes a power supply circuit having a rectifier circuit coupled to input terminals of the power supply circuit that are to be coupled to receive an AC input voltage. A switchmode power converter circuit is coupled to the rectifier circuit to receive a rectified input voltage to generate a regulated output voltage in response to the rectified input voltage. A switch is coupled between the rectifier circuit and the switchmode power converter circuit. A sense circuit is coupled to detect the AC input voltage. The switch is coupled to be switched in response to the sense circuit. The switch is coupled to be switched off when an absolute value of the AC input voltage exceeds a first threshold value. The switch is coupled to be switched on when the absolute value of the AC input voltage is below a second threshold value.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to power supplies, and morespecifically, the present invention relates to power supplies that havecapability to operate from a high voltage AC input voltage.

2. Background

In certain applications of AC/DC power supplies, it is sometimesdesirable for the power supply to operate outside the normal operatingvoltage range. Typically, AC/DC power supplies that are designed forworldwide operation are designed to operate with an AC input voltagebetween 85-265 VAC rms. However, in emerging markets such as India andChina, AC input voltages can be as high as 420 VAC for long periods oftime under certain conditions.

In the power supply, the AC input voltage is typically rectified by arectifier circuit to generate a DC voltage, which is applied to theinput of a switchmode power converter stage within the power supply. Arectified 420 VAC rms signal generates a peak DC voltage of almost 600V.This high DC voltage greatly increases the cost of the switchmode powerconverter circuit since all circuitry must be rated for this highvoltage condition. In particular, the input capacitors that are normallyrated for 400V for an 85-265 VAC rms supply must be increased in voltagerating to at least 600V. This is normally achieved by connecting two400V capacitors in series across the output terminals of the rectifiercircuit. In order to achieve the same effective capacitance in thisseries arrangement, each capacitor must also be double the capacitancevalue of the single 400V capacitor they are replacing.

The cost of the power supply circuit is therefore greatly increased inthis instance by adding the large additional input capacitors. The spacetaken by the additional capacitors required is also unacceptable in manyapplications such as AC/DC power supplies for cell phone chargingapplications where small enclosures and lightweight are keyrequirements.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a block diagram illustrating generally an example of a powersupply designed to receive an AC input voltage in accordance with theteachings of the present invention.

FIG. 2 is a block diagram illustrating generally an example of a highvoltage power supply circuit employing a switch to limit the voltageapplied to a switchmode converter stage of the power supply, with asense circuit sensing the voltage between the AC input terminals inaccordance with the teachings of the present invention.

FIG. 3 is a block diagram illustrating generally an example of a highvoltage power supply circuit employing a switch to limit the voltageapplied to a switchmode power converter stage of the power supply, withsense circuits sensing the voltage between the AC input terminals andthe voltage across input terminals of the switchmode power converter inaccordance with the teachings of the present invention.

FIG. 4A is a block diagram illustrating generally an example of a highvoltage power supply circuit employing a switch to limit the voltageapplied to a switchmode power converter stage of the power supply, witha sense circuit sensing the voltage at the output of the rectifiercircuit in accordance with the teachings of the present invention.

FIG. 4B shows generally example voltage waveforms and identifies themagnitude of key voltages during the circuit operation in accordancewith the teachings of the present invention.

FIG. 5 is a block diagram illustrating generally an example of a highvoltage power supply circuit employing a switch to limit the voltageapplied to a switchmode converter stage of the power supply, with sensecircuits sensing the voltage across the switch and the voltage acrossinput terminals of the switchmode power converter in accordance with theteachings of the present invention.

FIG. 6 shows a schematic illustrating generally an example of a highvoltage power supply circuit with a sense circuit sensing the voltagebetween the AC input terminals in accordance with the teachings of thepresent invention.

FIG. 7 shows a schematic illustrating generally an example of a highvoltage power supply circuit with a sense circuit sensing the voltage atthe output of the rectifier circuit.

DETAILED DESCRIPTION

Examples of apparatuses and methods for implementing a high voltagepower supply circuit are disclosed. In the following description,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone having ordinary skill in the art that the specific detail need notbe employed to practice the present invention. Well-known methodsrelated to the implementation have not been described in detail in orderto avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined for example into any suitablecombinations and/or sub-combinations in one or more embodiments.

A high voltage power supply circuit and method for implementing such acircuit in accordance with the teachings of the present invention willnow be described. Embodiments of the present invention involve methodsand apparatuses to generate high voltage power supply circuits.

FIG. 1 shows generally a block diagram of a power supply circuit 100. Asshown, power supply circuit 100 includes power supply input terminals101 and 102, which are coupled to receive an AC input voltage 191. Arectifier circuit 103 having input terminals 192 and 193 is coupled tothe power supply input terminals 101 and 102. The rectifier circuit 103has output terminals 120 and 121, which are coupled to the inputterminals 140 and 141 of a switchmode power converter 104. In theillustrated example, the rectifier circuit 103 could be one of severalcommonly used rectifier circuits such as for example, but not limitedto, a full wave rectifier circuit or a half wave rectifier circuit. Theswitchmode converter circuit 104 could be one of several commonly usedcircuit configurations such as for example, but not limited to, aflyback converter, a forward converter, a buck converter, SEPICconverter, a Cuk converter, or the like, and could employ any one of anumber of control schemes to regulate the output of the switchmodeconverter 104 at the output terminals 105 and 106. These control schemescould include, but are not limited to, voltage mode pulse widthmodulation (PWM), current mode PWM, on/off, hysteretic, resonant,quasi-resonant and self oscillating (switchmode converters using thistype of control are often referred to as RCC converters).

FIG. 2 is a block diagram showing generally one example of a powersupply circuit 200 in accordance with the teachings of the presentinvention. As shown, power supply circuit 200 includes input terminals201 and 202 that are coupled to receive an AC input voltage 291. Arectifier circuit 203 has input terminals 292 and 293 that are coupledto the power supply input terminals 201 and 202. The rectifier circuit203 has output terminals 220 and 221. Output terminal 220 is coupled toa first input terminal 240 of switchmode power converter 204. Outputterminal 221 is coupled to a first terminal 243 of a switch 207. Asecond input terminal 241 of switchmode converter 204 is coupled to asecond terminal 242 of switch 207.

As shown in FIG. 2, power supply circuit 200 also includes a sensecircuit 208, which is a voltage sense circuit coupled to sense or detectthe voltage between the power supply input terminals 201 and 202. In theexample, sense circuit 208 is coupled to a drive circuit 212, which iscoupled to drive the switch 207 on or off depending on the magnitude ofthe voltage sensed by sense circuit 208. In the illustrated example, theswitch 207 is coupled to be off when the absolute value of the voltage291 between the input terminals 201 and 202 exceeds a first thresholdvalue. In one example, the first threshold value is determined by thedesign of sense circuit 208. Furthermore, the sense circuit is coupledto the switch 207 such that the switch is on when the absolute value ofthe voltage 291 between the input terminals 201 and 202 is below asecond threshold value. In the example, the switch 207 is off for theduration of the period between the time where the absolute value of thevoltage 291 between the input terminals 201 and 202 exceeds the firstthreshold value and the time when the absolute value of the voltage 291between the input terminals 201 and 202 goes below a second thresholdvalue. During this off state, the switch 207 is not periodicallyswitched on and off and is not in a current limit state but is in an offstate for the complete time.

In one example, the first and second threshold values may besubstantially equal for simplicity of the sense circuit 208. However, inanother example, there may be some hysteresis in the input voltagevalues in which the first threshold voltage level is higher than thesecond voltage threshold level. Such an example will be discussed inmore detail below in connection with the examples shown in FIGS. 4A and4B. Referring back to the example shown in FIG. 2, the voltage betweenfirst and second terminals 243 and 242 of switch 207 when the switch 207is off is substantially equal to the difference between the absolutevalue of the voltage between the power supply input terminals 201 and202 and the voltage between the first and second terminals 240 and 241of the switchmode converter 204. In this example, the voltage betweenthe input terminals 240 and 241 of switchmode converter 204 neverexceeds the first threshold voltage value set in sense circuit 208. Inconditions where the input voltage 291 exceeds this first thresholdvalue therefore, the switch 207 will protect the switchmode converter204 and instead the excess voltage will be dropped across switch 207while it is off.

It is noted that a capacitor often referred to as a bulk capacitor ofthe switchmode converter that is coupled between input terminals 240 and241 is not shown but may be included in the example illustrated in FIG.2. The bulk capacitor stores energy for the switchmode converter 204 tocontinue operating while switch 207 is off. As such, the switchmodepower converter 204 will continue to operate normally whether switch 207is on or off as long as the period of the off state is short enough thatthe switchmode converter 204 bulk capacitor is still sufficientlycharged for the switchmode converter 204 to operate. This is typicallyeasily achieved since the AC input voltage between input terminals 201and 202 exceeds the first threshold voltage level and remains above thesecond threshold level for a short period of typically only a fewmilliseconds for each half cycle of the input AC voltage 291. Theexample shown in FIG. 2 therefore allows the switchmode converter 204 tobe designed only for input voltages up to the first threshold voltagelevel since the switch 207 is coupled to limit the maximum voltagebetween the first and second input terminals 240 and 241 of theswitchmode power converter 204 when the voltage between the power supplyinput terminals 201 and 202 exceeds a threshold value.

In one example, the sense circuit 208 in FIG. 2 may be referenced to theoutput terminal 221 of the rectifier circuit, as shown by the dashedline connection 223. However, since the voltage between rectifiercircuit 203 output terminal 221 and input terminal 293 is typically 1volt or less when current is flowing from output terminal 221 to inputterminal 293 with a standard diode rectification circuit, the referencefor sense circuit 208 can also be taken from the input terminal 202 asshown by connection 222 in FIG. 2.

In one example, the switch 207 shown in the block diagram of FIG. 2 willbe on continuously when the absolute value of the input voltage 291 isbelow the first and second threshold voltage levels. In a typical powersupply design, the first and second threshold voltage levels will be setsomewhere above the normal operating AC input voltage of thegeographical region where the power supply 201 is to be used. Forexample, in a geography when the nominal AC line voltage is 230 VAC rms,the sense circuit 208 might be designed such that the first thresholdlevel is at an absolute value of input voltage 291 of 450V, which is theapproximate peak voltage of a 318 VAC rms AC input voltage. In theexample, the second threshold level might then be set at a voltage levelless than or equal to the first threshold voltage level, such as forexample at 425V. Therefore, in this example, only when the AC inputvoltage 291 rises above 318 VAC rms will switch 207 be turned from an onstate to an off state for the portion of the AC input waveform where theabsolute value of voltage 291 exceeds the first threshold level of 450 Vand remains above the second voltage threshold level of 425V, inaccordance with the teachings of the present invention. In an example,the type of switch that may be employed for switch 207 is very flexibleand can be a semiconductor switch such as a bipolar transistor, a metaloxide semiconductor field effect transistor (MOSFET) or an insulatedgate bipolar transistor (IGBT).

In addition, the frequency at which the switch will be turned on and offis relatively low, as AC line frequencies are typically in the range of47-60 Hz depending on the geographical region. Thus, it is feasible thata semiconductor switch, such as a gate turn off thyristor, or even amechanical switch, such as a relay, could be used in some examples forswitch 207 in accordance with the teachings of the present invention.Regardless of the type of switch used for switch 207, the voltage acrossthe switch 207 when it is on can be regarded as substantially zero sinceit will be so much smaller than the magnitude of the input voltage 291.When the switch 207 is off, substantially zero current flows between theswitch 207 terminals 242 and 243. The voltage between input terminals240 and 241 of switchmode power converter 204 is therefore unregulatedwhen the switch 207 is off.

FIG. 3 shows generally a block diagram of another example of a powersupply circuit 300 in accordance with the teachings of the presentinvention. The example power supply circuit 300 of the block diagram ofFIG. 3 shares many aspects with the example power supply circuit 200 ofthe block diagram of FIG. 2. However, in example power supply circuit300, the example sense circuit is split into multiple portions, whichare labeled in FIG. 3 as sense circuit 308 and 309. As shown, theportion of the sense circuit labeled sense circuit 308 is coupled to thepower supply input terminals 301 and 302 while the portion of the sensecircuit labeled sense circuit 309 is coupled to the input terminals 340and 341 of switchmode power converter circuit 304. In the example, avoltage between the input terminals 340 and 341 of the switchmode powerconverter circuit 304 is substantially equal to the absolute value ofthe AC input voltage between the power supply input terminals 301 and302.

In the example, the voltage appearing across input terminals 340 and 341of switchmode power converter 304 is directly sensed by sense circuitportion 309, which provides a signal 310 to drive circuit 312 in orderto switch the switch 307 from an on state to an off state when thevoltage between the first and second input terminals 340 and 341 ofswitchmode converter 304 exceeds a first threshold value. In theexample, the switch 307 is coupled to be switched on when the absolutevalue of the voltage between the power supply input terminals 301 and302 sensed by sense circuit portion 308 is below a second thresholdvalue. Although the voltage sensing to determine when switch 307 is tobe turned from an on state to an off state in FIG. 3 is performed at adifferent locations by sense circuit portions 308 and 309 compared tothe power supply 200 example illustrated in FIG. 2, the result is thesame in that the maximum voltage between the input terminals 340 and 341of switchmode converter 304 is limited in accordance with the teachingsof the present invention.

FIG. 4A shows generally a block diagram of another example of a powersupply circuit 400 in accordance with the teachings of the presentinvention. As shown, the example power supply circuit 400 of the blockdiagram of FIG. 4A shares many aspects with the example power supplycircuits 200 and 300 shown in FIG. 2 and FIG. 3. However, in powersupply circuit 400 of FIG. 4, the sense circuit 408 is coupled to sensethe voltage between the output terminals 420 and 421 of therectification circuit 403. In the example, the switch 407 is thereforecoupled to be off when the voltage between the first and second outputterminals 420 and 421 of the rectifier circuit 403 exceeds a firstthreshold value in accordance with the teachings of the presentinvention.

In the example, the switch 407 is coupled to be on when the voltagebetween first and second output terminals 420 and 421 of the rectifiercircuit 403 is below a second threshold value. In addition, since therectification circuit 403 is typically a simple diode bridge having verylow voltage drop in the forward direction of typically less than 2 V,for the purposes of the power supply 400 operation, the voltage betweenthe output terminals 420 and 421 of the rectification circuit 403 canalso be regarded as substantially equal to the absolute value of the ACinput voltage between power supply input terminals 401 and 402 inaccordance with the teachings of the present invention.

FIG. 4B illustrates generally typical waveforms 480 that would beobserved during the operation of example power supply 400. In thisexample, first and second threshold voltage levels are assumed equal asa single voltage threshold level 487 as shown. In an example withhysteresis, the first and second thresholds would be different. As shownin the illustration, the AC input voltage waveform 491 exceeds thethreshold voltage level 487 for a time period 481 during each half cycleof the AC input voltage waveform 491. During period 481, switch 407 inFIG. 4A is off. For the remainder of the time, switch 407 can be on.

In one example, whether switch 407 actually remains on at all timesother than period 481 in FIG. 4B depends on the design of sense circuit408 and drive circuit 412 in FIG. 4A. For example during period 488 inFIG. 4B, input voltage waveform 491 is at a very low voltage duringperiod 489. The sense circuit 408 and drive circuit 412 may not havesufficient supply voltage to remain operational. If this is the case,drive circuit 412 may not be able to maintain switch 407 in an on stateand switch 407 may transition to an off state until the absolute valueof supply voltage waveform 491 is sufficient to sustain operation ofsense circuit 408 and drive circuit 412. It is appreciated that even ifswitch 407 does transition to an off state for the above reasons duringa period 489 when it would otherwise be in an on state, at least some ofthe benefits provided by the power supply circuit are retained inaccordance with the teachings of the present invention. The reason isthat during this period 489, no current is flowing into switchmode powerconverter circuit 404 input terminal 440 in any case since the value ofVsin 462 during period 489 is greater than the absolute value of inputvoltage 491. During the switch 407 off time 481, the voltage acrossfirst and second terminals 443 and 442 of switch 407, or Vsw 461, issubstantially equal to the difference between the rectifier circuit 403output voltage Vrout 460 and the switchmode power converter 404 inputvoltage Vsin 462. During the switch 407 off time 481 there is no currentflow in rectifier circuit 403, the rectifier output voltage 460 in FIG.4A is then substantially equal to the absolute value of the AC inputvoltage 491.

The rectifier output voltage 460 is therefore representative of themagnitude of the voltage between the power supply input terminals 401and 402. In FIG. 4A therefore, when switch 407 is off, Vsw 461 issubstantially equal to the difference between the absolute value of thevoltage 491 between the power supply input terminals 401 and 402 and thevoltage 462 between first and second input terminals 440 and 441 of theswitchmode power converter 404. The waveforms of FIG. 4B also show thefact that Vsin 462 drops during the switch 407 off time 481. This isbecause Vsin 462 is unregulated during period 481. The slope of thevoltage waveform 486 during period 481 is determined by the voltageacross a bulk capacitor coupled between terminals 440 and 441 internalto the switchmode power converter 404 and the loading at the outputterminals 405 and 406, which is described in more detail below in theschematic diagrams of FIG. 6 and FIG. 7.

FIG. 5 shows generally a block diagram of another example of a powersupply circuit 500 in accordance with the teachings of the presentinvention. The example power supply circuit 500 of FIG. 5 shares aspectswith the power supply circuits 200, 300 and 400 of FIGS. 2, 3 and 4.However, in power supply circuit 500 of FIG. 5, the voltage appearingacross input terminals 540 and 541 of switchmode power converter 504 isdirectly sensed by sense circuit portion 509, which provides a signal510 to drive circuit 512 in order to switch the switch 507 from an onstate to an off state when the voltage between the first and secondinput terminals 540 and 541 of switchmode converter 504 exceeds a firstthreshold value.

As shown in the depicted example, switch 507 is coupled to be switchedfrom an off state to an on state when the voltage across switch 507 isbelow a second threshold value when the switch 507 is off. As shown inthe illustrated example, sense circuit portion 508 senses the voltageacross switch 507 and provides a signal 511 to drive circuit 512 toachieve this functionality. Although the voltage sensing to determinewhen switch 507 is to be switched from an off state to an on state inFIG. 5 is performed at a different location within the power supplycircuit 500 than in power supply circuit 300 of FIG. 3, the result isthe same in that the maximum voltage between the input terminals 540 and541 of switchmode converter 504 is limited.

FIG. 6 is a schematic that shows generally one example of a power supplycircuit 600 in accordance with the teachings of the present invention.It is noted that the example power supply circuit 600 shown in FIG. 6has similarities to the example power supply circuit 200 shown in FIG.2. As shown in the example illustrated in FIG. 6, power supply circuit600 includes a switchmode power converter 640 including a TinySwitchpower converter device, available from Power Integrations, Inc., of SanJose, Calif. In the example, sense circuit 608 senses the voltagebetween input terminals 601 and 602 and output terminal 621 of rectifiercircuit 603, which is equivalent to the connection 223 to outputterminal 221 of rectifier circuit 203 in FIG. 2. In the illustratedexample, sense circuit 608 is an AC voltage sense circuit since it isdirectly coupled to the AC input terminals 601 and 602. Diodes D5 and D6rectify the voltage signals on input terminals 601 and 602 and applythis voltage to the resistor divider formed by resistors R1 and R2. Whenthe voltage across resistor R2 exceeds the base emitter thresholdvoltage of transistor Q1, signal 611 turns on transistor Q1, which formspart of drive circuit 612. The collector of transistor Q1 is pulled low,which in turn pulls the gate of switch 607 low turning off switch 607.In the illustrated example, drive circuit 612 also includes resistor R3,which provides a pull up signal to the gate of switch 607 whentransistor Q1 is off, to turn switch 607 on. Zener diode VR1 provides aclamp that limits the maximum pull up voltage applied to the gate ofswitch 607.

In the example shown in FIG. 6, rectifier circuit 603 is a full waverectifier. The output terminals 620 and 621 of rectifier circuit 603 arecoupled to first terminal 640 of switchmode converter 604 and to firstswitch terminal 643 of switch 607. A second input terminal 641 ofswitchmode converter 604 is coupled to a second terminal 642 of switch607. The example schematic of FIG. 6 shows the bulk capacitor C1 ofswitchmode converter 604 as discussed earlier. Capacitor C1 provides theenergy storage that sustains the operation of switchmode converter 604during the period that switch 607 is off. In one example, capacitor C1is used during the operation of switchmode power converter 604 even whenswitch 607 is on. This is because as shown in FIG. 4B for the period488, where the input AC voltage 491 is less than the bulk capacitorvoltage 486, the switchmode power converter 604 continues to operate.This bulk capacitor C1 is therefore used even in standard circuits whereswitch 607 is not used and, therefore, there is no cost penaltyassociated with the use of bulk capacitor C1 in accordance with theteachings of the present invention.

FIG. 7 is a schematic showing generally another example of a powersupply circuit 700 in accordance with the teachings of the presentinvention. It is noted that the example power supply circuit 700 shownin FIG. 7 has similarities with the example power supply circuit 400shown in FIG. 4A. As shown in the example illustrated in FIG. 7, powersupply circuit 700 includes a switchmode power converter 704 including aTinySwitch power converter device, available from Power Integrations,Inc., of San Jose, Calif. In the example, sense circuit 708 senses thevoltage between output terminals 720 and 721 of rectifier circuit 703and applies a signal 711 to drive circuit 712, which drives switch 707on and off according to the operation of the sense circuit 708 similarto that as described above for example with reference to power supplycircuit 600 of FIG. 6.

As shown in the example of FIG. 7, power supply circuit 700 also shows acomponent 790 coupled across switch 707, which is a protective clampcomponent to limit the maximum voltage across switch 707 when switch 707is off. Clamp component 790 may be used in some implementations of thepower supply circuit 700 where for example lightning surge voltage testsare carried out under conditions when switch 707 is off. Under theseconditions, a high voltage surge is typically applied between AC inputterminals 701 and 702. Typical surge voltages are in the range of 1000to 2000V. Since the voltage across switch 707 in the off state is thedifference between the voltage between input terminals 701 and 702 andthe voltage between input terminals 740 and 741 of switchmode converter704, clamp component 790 is included in the example to help avoid damageto switch 707.

In the example, when a threshold voltage determined by the choice ofclamp component 790 is reached across switch 707, the clamp component790 allows current to flow through clamp component 790 and the bulkcapacitor C1. Bulk capacitor then absorbs the surge energy with verylittle additional voltage rise across capacitor C1, which allows powersupply circuit 700 to safely meet surge voltage withstand testing.Although the illustrated example shows as a Zener diode in FIG. 7,component 790 could be a metal-oxide-varistor or other semiconductorclamp component or even split into a resistor and capacitor snubbernetwork, or other suitable technique in accordance with the teachings ofthe present invention.

Other protective components in certain example applications of powersupply 700 could be the use of an optional resistive element (not shown)coupled between the first terminal 743 of switch 707 and the outputterminal 721 of rectifier circuit 703, which would limit in rush currentduring power supply turn on at high input voltage.

In the illustrated example, resistor R5 in sense circuit 708 is anoptional component that creates hysteresis in sense circuit 708 toseparate the first voltage threshold level and the second voltagethreshold levels, as described previously. For example, when the firstvoltage threshold level, determined by sense circuit 708 is reached,transistor Q1 turns on and switch 707 is switched off as describedabove. When this happens, the voltage across switch 707 will rise,feeding current through resistor R5 to further increase the currentflowing into the base of transistor Q1. The connection of resistor R5will therefore cause the voltage between rectifier circuit 703 outputterminals 720 and 721 to be lower than the first threshold voltage levelfor transistor Q1 to be switched from an on state to an off state. Thisin turn will cause the second voltage threshold level to be lower thanthe first voltage threshold level and therefore introduce somehysteresis into the operation of voltage sense circuit 708. It isappreciated that this is only one example of a low cost implementationof hysteresis and that there are many other circuits that could be usedto implement hysteresis in accordance with the teachings of the presentinvention.

It is appreciated that the examples power supply circuits 600 and 700illustrated in FIGS. 6 and 7 share a similarity in that no external biascircuitry is added for sense circuits 608 and 708 to provide the addedfunction in accordance with the teachings of the present invention.Therefore, there is no requirement to power the sense circuits 608 and708, switches 607 and 707 or drive circuits 612 and 712 from the outputof the switchmode power converters 604 and 704 in accordance with theteachings of the present invention. There is also no requirement for anybiasing means coupled between first and second terminals of switches 607and 707. Indeed, in the example, the drive current required to driveswitch 607 in FIG. 6 is derived entirely from the AC input voltagecoupled to be received between input terminals 601 and 602 and thevoltage between and output terminals 620 and 621 of rectifier circuit603 in accordance with the teachings of the present invention. In theexample shown in FIG. 7, the drive current required to drive switch 707is derived entirely from the voltage between and output terminals 720and 721 of rectifier circuit 703 in accordance with the teachings of thepresent invention. Furthermore, there is no requirement in any of theexamples for a solenoid to trip or activate the operation of any of thecircuitry in FIG. 6 or FIG. 7 in accordance with the teachings of thepresent invention.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A power supply circuit, comprising: a rectifier circuit coupled toinput terminals of the power supply circuit to be coupled to receive anAC input voltage; a switchmode power converter circuit coupled to therectifier circuit to receive a rectified input voltage to generate aregulated output voltage in response to the rectified input voltage; aswitch coupled between the rectifier circuit and the switchmode powerconverter circuit; and a sense circuit coupled to detect the AC inputvoltage, wherein the switch is coupled to be switched in response to thesense circuit, wherein the switch is coupled to be switched off when anabsolute value of the AC input voltage exceeds a first threshold valueand wherein the switch is coupled to be switched on when the absolutevalue of the AC input voltage is below a second threshold value.
 2. Thepower supply circuit of claim 1 wherein the sense circuit is coupled tothe input terminals of the power supply circuit to detect the AC inputvoltage.
 3. The power supply circuit of claim 1 wherein the sensecircuit is coupled to input terminals of the switchmode power convertercircuit to detect when the absolute value of the AC input voltageexceeds the first threshold value to turn off the switch and wherein thesense circuit is coupled to the input terminals of the power supplycircuit to detect when the absolute value of the AC input voltage isbelow the second threshold value to turn on the switch.
 4. The powersupply circuit of claim 3 wherein a voltage between input terminals ofthe switchmode power converter circuit is substantially equal to theabsolute value of the AC input voltage.
 5. The power supply circuit ofclaim 1 wherein the sense circuit is coupled to output terminals of therectifier circuit to detect the absolute value of the AC input voltage.6. The power supply circuit of claim 5 wherein a voltage across theoutput terminals of the rectifier circuit is substantially equal to theabsolute value of the AC input voltage.
 7. The power supply circuit ofclaim 5 wherein a voltage across the output terminals of the rectifiercircuit is representative of a magnitude of the AC input voltage.
 8. Thepower supply circuit of claim 5 wherein the switch is coupled to beswitched from on to off to limit a maximum voltage between inputterminals of the switchmode power converter in response to the sensecircuit.
 9. The power supply circuit of claim 1 wherein the sensecircuit is coupled to input terminals of the switchmode power convertercircuit to detect when the absolute value of the AC input voltageexceeds the first threshold value to turn off the switch and wherein thesense circuit is coupled across the switch to detect when the absolutevalue of the AC input voltage is below the second threshold value toturn on the switch.
 10. The power supply circuit of claim 9 wherein avoltage across the switch when the switch is off is substantially equalto a difference between the absolute value of the AC input voltage and avoltage between input terminals of the switchmode power converter. 11.The power supply circuit of claim 1 wherein a voltage across the switchwhen the switch is on is substantially zero.
 12. The power supplycircuit of claim 1 wherein the first and second voltage threshold valuesare substantially equal.
 13. The power supply circuit of claim 1 whereinthe first voltage threshold value is greater than the second voltagethreshold value.
 14. The power supply circuit of claim 1 wherein theswitchmode power converter is one of a flyback converter, a forwardconverter, a buck converter, a SEPIC converter or a Cuk converter. 15.The power supply circuit of claim 1 wherein the switch comprises asemiconductor switch.
 16. The power supply circuit of claim 15 whereinthe semiconductor switch comprises one of a bipolar transistor, a metaloxide semiconductor field effect transistor (MOSFET), an insulated gatebipolar transistor (JGBT) or a gate turn off thyristor.
 17. The powersupply circuit of claim 1 wherein the switch comprises a mechanicalswitch.
 18. The power supply circuit of claim 17 wherein the mechanicalswitch comprises a relay.
 19. The power supply circuit of claim 1further comprising a drive circuit coupled between the sense circuit andthe switch, the drive circuit coupled to drive the switch on or off inresponse to the sense circuit.
 20. The power supply circuit of claim 1wherein a voltage between input terminals of the switchmode powerconvener is unregulated when the switch is off.
 21. The power supplycircuit of claim 1 wherein the rectifier circuit includes first andsecond outputs and the switchmode power converter circuit includes firstand second inputs, wherein the first output of the rectifier circuit iscoupled to the first input of the switchmode power converter circuit andwherein the switch is coupled between the second output of the rectifiercircuit and the second input of the switchmode power converter circuit.22. A method of regulating a power supply, comprising: rectifying an ACinput voltage received at input terminals of a power supply circuit witha rectifier circuit; generating a regulated output voltage at outputterminals of the power supply circuit with a switchmode power convertercircuit coupled to receive a rectified input voltage from the rectifiercircuit; switching off a switch coupled between the rectifier circuitand the switchmode power converter circuit when an absolute value of theAC input voltage exceeds a first threshold value; and switching on theswitch coupled between the rectifier circuit and the switchmode powerconverter circuit when the absolute value of the AC input voltage isbelow a second threshold value.
 23. The method of regulating the powersupply of claim 22 further comprising sensing the AC input voltage atthe input terminals of the power supply circuit with a sense circuit.24. The method of regulating the power supply of claim 22 whereinswitching off the switch coupled between the rectifier circuit and theswitchmode power converter circuit comprises sensing the AC inputvoltage at input terminals of the switchmode power converter and whereinswitching on the switch coupled between the rectifier circuit and theswitchmode power converter circuit comprises sensing the AC inputvoltage at the input terminals of the power supply circuit.
 25. Themethod of regulating the power supply of claim 22 further comprisingsensing the absolute value of the AC input voltage at output terminalsof the rectifier circuit with a sense circuit.
 26. The method ofregulating the power supply of claim 25 wherein a voltage across theoutput terminals of the rectifier circuit is representative of amagnitude of the AC input voltage.
 27. The method of regulating thepower supply of claim 25 further comprising limiting a maximum voltagebetween input terminals of the switchmode power converter.
 28. Themethod of regulating the power supply of claim 22 wherein switching offthe switch coupled between the rectifier circuit and the switchmodepower converter circuit comprises sensing the AC input voltage at inputterminals of the switchmode power converter and wherein switching on theswitch coupled between the rectifier circuit and the switchmode powerconverter circuit comprises sensing a voltage across the switch.
 29. Themethod of claim 28 wherein the voltage across the switch when the switchis off is substantially equal to a difference between the absolute valueof the AC input voltage and a voltage between input terminals of theswitchmode power converter.
 30. The method of claim 22 wherein the firstand second voltage threshold values are substantially equal.
 31. Themethod of claim 22 wherein the first voltage threshold value is greaterthan the second voltage threshold value.
 32. The method of claim 22further comprising driving the switch with a drive circuit whenswitching the switch.